RJH explained :
Can anyone identify this noise? -

https://drive.google.com/open?id=1o_...flq4qiwX_jS6vh

It's a sort of high pitch metallic rattling, happens all the time, and has
started since I drained and refilled the closed combi system.

Water circulation pump, if it has one fitted inside the case, fan, or
simply something inside the casing loose and rattling.